High resolution measurement of light frequency or wavelength typically makes use of comparing photon wavelength to another object. One method is to compare the wavelength of light to a physical length, such as the cavity length of a Fabry Perot interferometer. Another method is to compare photon wavelength to the wavelength of other photons, called heterodyne detection. A third method is to compare the energy of a photon, which is proportional to frequency, to an absorption line. A new method is introduced here that measures photon wavelength by an absorption line that relies upon dispersion, which accompanies absorption, of an absorption line.